The present invention relates generally to antistatic devices and more particularly to a fabric and body strap having both conductive and elastomeric properties.
Generally a need exists for devices to control the elastrostatic charge accumulation on the body or person of an individual. Certain individuals occupy areas or handle materials in which an electrostatic discharge could either be hazardous to the individual or could damage the material being handled. Examples are individuals in the proximity of an explosive or hazardous environment and individuals who must handle static sensitive electronic components.
Many devices have been developed to solve the problem of electrostatic charge accumulation and subsequent discharge. These include devices which have been fashioned into body straps or wrist straps to be worn by the particular individual involved. These body straps or wrist straps are then adapted to be connected to a ground potential, possibly through a predetermined resistance, in order to dissipate the electrostatic charge accumulation on the individual and to prevent additional electrostatic charge accumulation. These devices operate by draining off any accumulated electrostatic charge from the individual.
One of these devices is the Model 2064 wrist strap manufactured by Minnesota Mining and Manufacturing Company (3M ), St. Paul, Minn. The Model 2064 wrist strap consists of a Velostat (Velostat is a trademark of the 3M Company) strip held on the wrist with a band of nylon. Velostat conductive material is a carbon loaded conductive polymer. The operation of the wrist strap relies on the conductive polymer to conduct electrostatic charge via the individual's wrist to a ground cord secured to the wrist strap with an electrically conductive snap connection. The wrist strap relies on a hook and loop fastener system (e.g. Scotchmate, a trademark of 3M, fastener or Velcro, a trademark of Velcro U.S.A., Inc., Manchester, New Hampshire, fastener) to secure the wrist strap to the wrist of the individual wearer.
A wrist strap manufactured by Semtronics Corporation, Peach Tree City, Ga. is constructed from similar functional components. The Semtronics wrist strap also uses a black conductive plastic secured to the wrist with a hook and loop closure system.
A wrist strap manufactured by Simco, Landsdale, Penna., also uses a similar system. The entire band of the wrist strap is made of a nylon hook and loop fastener system. The Simco wrist strap has a carbon loaded conductive material secured to the inner surface of the hook and loop fastener. A snap connection is provided for a ground cord. The Simco wrist strap again relies on the conductive polymer for conducting the electrostatic accumulation on the individual to the snap connection and to the grounding cord. Similarly, the Simco wrist strap also relies on the hook and loop fastener for the closure system.
A wrist strap manufactured by Wescorp of Mountain View, Calif. consists of a carbon loaded conductive fabric with a hook and loop fastener. The Wescorp wrist strap relies on the conductive fabric for the conduction of electrostatic charge from the individual instead of the conductive polymer as in the previous straps but again relies on the hook and loop fastener for the closure system. The Wescorp strap also utilizes the conductive fabric for a connection to ground rather than a connection point to a ground wire.
A strap manufactured by Walter G. Legge Company, New York, N. Y., carrying the name "WRISTSTAT" uses a black nylon band with a hook and loop fastener. A conductive polymer is attached to the band with a metallic plate at a relatively narrow location around the strap. The conductive polymer also has a snap connection to a ground cord. The Legge wrist strap relies on the metal plate and the conductive polymer for conductivity and relies on the nylon band with the hook and loop fastener for the closure system.
The straps heretofore described are all very similar in nature. Almost all rely on a carbon loaded conductive polymer and the remainder on a carbon loaded fabric. All of the wrist straps rely on a hook and loop fastener for a closure system. The use of a carbon loaded conductive material, while electrically functional, yields a structure black in color which is aesthetically not appealing. Although it must be noted that the Simco wrist strap limits the black color to the interior surface of the nylon band which otherwise can be colored as desired. Most importantly, however, all of the wrist straps rely on a closure system which is detachable and needs to be adjusted by the individual wearer. All of the straps rely on the firmly intimate contact of a conductive member to the body or person of the individual. A detachable and adjustable closure system must be adjusted individually by the wearer each time the strap is put on. Proper adjustment is required for proper functioning of the strap. This adjustability necessarily means that since it can be adjusted properly it can also be adjusted improperly. Proper functioning then demands that the wearer be trained and skilled in the proper adjustment and have the incentive to adjust the strap properly every time it is worn. In any event, the wearer's supervisors can never be sure of proper installation and proper performance consistently. Particularly in an area where sensitive electronic components are being handled, the result may be a degradation of component reliability since the improper adjustment and loss of function of the strap may result in damage to the sensitive electronic components which may not become apparent until installed and used by the ultimate customer of those components.
Wescorp also has a strap consisting of a metallic bead chain to which an electrical ground cord is slideably attached. The strap relies on the metallic beads for conductivity. Since it is worn loosely around the wrist, it can be made large enough to slip over the hand onto the wrist and thus no detachable closure is required. This strap however does suffer from the same improper adjustment problems of previous straps since its electrical connection is not ensured since the strap does not intimately contact the body (wrist) of the individual wearer.
Controlled Static Company, Santa Fe, Calif. manufactures a wrist strap known in the trade as a Fred strap. The strap is a metallic expansion band having a snap connection for an electrical ground cord. The band is reminiscent of a metallic expansion watchband. The band relies on the conductivity of the metal for the drainage of the accumulated electrostatic charges and will expand to slip on the wrist over the hand and then fit relatively snugly. However, the wrist strap suffers the disadvantage of a relatively low expansion ratio. The strap must be large enough to slip over the hand, yet small enough to fit snugly on the wrist.
The two previous metallic straps also suffer another significant disadvantage. Since the highly conductive metallic surface is available at the outside surface of the wrist strap, there is a danger of accidental contact with a high voltage source and the resultant "welding" of the strap to that source preventing disengagement of the wearer from the high voltage source. It is for this reason that some electricians do not wear metallic rings, bracelets and other jewelry.